arXiv subjects

Yousef Bisabr

Publications and source records attributed to Yousef Bisabr.

At least 19 recordsLinked to original sources

arXiv2025

Probing Scalar-Photon Coupling in the Early Universe: Implications for CMB Temperature and Anisotropies

The Hubble tension, as a persistent discrepancy between early-time and late-time measurements of the Hubble constant, motivates explorations of new physics in the early Universe. In a recent early dark energy (EDE) model, we introduced a scalar field interacting with the radiation sector at early-time before recombination. We showed that such a scalar-photon coupling can lead to an accelerated expansion phase in which the energy density of scalar component dilutes faster than radiation does, a crucial feature for a successful EDE model. In the present work, we extend our analysis to investigate how this scalar-photon coupling affects the CMB temperature-redshift law and CMB anisotropies. We demonstrate that the temperature-redshift law deviates from the standard relation $T(z)\propto (1+z)$ due to the scalar-photon coupling. This deviation is controlled by a model parameter $ε$, which quantifies the rate of energy transfer between the scalar field and radiation. We also argue that a positive value of $ε$ shifts the acoustic peaks to larger scales, which potentially alleviates the Hubble tension. These findings suggest that scalar-photon coupling is a testable mechanism for reconciling different cosmological observations.

astro-ph.CO↗

arXiv2025

Interacting Early Dark Energy

We explore a model of interacting Early Dark Energy (EDE) in which a minimally coupled scalar field, representing the EDE, interacts with the radiation sector through an exponential coupling function in the radiation-dominated era. This framework can be inspired by modified theories of gravity, including $f(R)$ gravity, the Einstein frame representation of Brans-Dicke theory, and chameleon gravity. Our findings reveal that the traditional law of radiation conservation is altered to $ρ_γ\propto a^{-4+ε}$, where the parameter $ε$ measures the rate of energy transfer between radiation and EDE. Assuming a constant energy transfer, we show that the scalar field behaves as $ϕ\propto\ln a$, indicating that $ϕ$ diverges as $a$ approaches zero. Additionally, we demonstrate that the interacting scalar-photon system behaves similar to an effective cosmological constant in the early stages of evolution of the Universe. Moreover, by solving the conservation equation associated with the scalar field, we derive an analytical expression for the ratio $r=ρ_ϕ/ρ_γ$. Our results indicate that $r$ diminishes as the Universe expands, which is essential for a successful EDE model. Our investigation into the parameter space confirms that the expected behavior of $r$ during recombination aligns with contemporary cosmological data. These insights underscore crucial aspects necessary for any feasible EDE model and present exciting possibilities for resolving the Hubble tension.

gr-qc↗

arXiv2025

Can Brans-Dicke Scalar Field Mimic Early Dark Energy?

We show that a generalisation of Brand-Dicke (BD) theory can provide a theoretical basis for dark energy at early times. This generalisation is based on introducing a potential for the BD scalar and allowing of the latter to interact with radiation in the radiation-dominated rea. We argue that the model we take up provides a strong theoretical basis for early dark energy (EDE) so that the scalar field can mimic a cosmological constant and quintessence behaviors under slow-roll and non-slow-roll conditions, respectively.

hep-th↗

arXiv2023

Attractor Solutions in Interacting Dark Energy Models

We investigate a cosmological model in which dark energy, represented by a quintessential scalar field, is coupled to a dark-matter perfect fluid in the spatially flat Friedmann-Robertson-Walker Universe. This allows an energy exchange in the dark sector which could happen both at early times before recombination era or at late times. We use the coupling function $Q=γρ_{dm}\dotφ$ which is induced by conformal transforming scalar-tensor and $f(R)$ gravity theories to Einstein frame. It is argued that there is a connection between this coupling function and $Q\propto ρ_{dm}H$. A dynamical analysis is used to show that there are early- and late-time attracting solutions for which the system evolves for a wide range of initial conditions. These attractors generalize the scaling solutions which have been already found in the non-interacting case.

gr-qc↗

arXiv2022

The Need for Curriculum Updating in Physics Education

We argue that most advances in science and technology during the past and the present centuries rely on modern physics concepts developed in the early $20$th century. The reliance is so profound that any improvement of literacy of science and technology falters in the absence of those concepts in physics education. Despite this important role, there are some remarks in literature concerning that modern topics have gone unnoticed in physics textbooks \cite{cht}. We discuss confirmation of this claim and also argue that physics education without reference to the difficulties of classical concepts and the need for paradigm change can lead to serious misconceptions and gives an incorrect image of science.

physics.ed-ph↗

arXiv2021

Inflation and Cosmological Constant

In construction of an inflationary model, one usually assumes that the matter sector of the gravitational action is minimally coupled to the background. It means that the matter (inflaton) part of the action is coupled with the same metric of the gravitational part. We elaborate on this assumption and investigate some of the consequences. We assume that the gravitational and the inflaton sectors belong to different units (or conformal frames). We show that this coupling can convert a single-field inflationary model into a two-field one with a mixed kinetic term. The energy-momentum tensor of the inflaton is therefore non-conserved due to the interaction with the conformal factor. This allows an energy exchange between the two fields and provides us with a mechanism for reduction of a large effective cosmological constant during inflation.

gr-qc↗

arXiv2021

Redshift Dependence of CMB Temperature in BSBM $α$-Varying Theories

We have investigated some cosmological consequences of variation of the fine structure constant, $α$, in the context of the Bekenstein-Sandvik-Barrow-Magueijo (BSBM) model. In this context, such a variation is attributed to electric charge by letting the latter take on the value of a real scalar field, $ϕ$, which varies generally in space and time. We consider the dynamics of $ϕ$ in the case that it is allowed to have an exponential potential. We have found solutions for evolutions of $ϕ$ and $α$ in the radiation and the matter dominated eras. By employing observational bounds on the temporal variation of $α$, we constrain the model parameters. We then investigate the impact of $α$-variation on redshift dependence of CMB temperature. It is shown that there is a deviation from the standard evolution of the temperature which contains two different contributions, one is important at high redshift eras and the other is mostly effective at low redshift eras.

gr-qc↗

arXiv2020

On Non-triviality of Units Transformations

There are some theoretical arguments about possible variations of fundamental constants with cosmic time. We review the fact that all conversion factors depend on these quantities and consider how their variations may affect transformations of units. We deal with the relevance of the issue to the cosmological constant problem.

gr-qc↗

arXiv2020

Gravitational Coupling, Dynamical Changes of Units and The Cosmological Constant Problem

A spacetime interval connecting two neighbouring points can be measured in different unit systems. For instance, it can be measured in atomic unit defined in terms of fundamental constants existing in quantum theories. It is also possible to use a gravitational unit which is defined by using of properties of macroscopic objects. These two unit systems are usually regarded as indistinguishable up to a constant conversion factor. Here we consider the possibility that these two units are related by an epoch-dependent conversion factor. This is a dynamical changes of units. Regarding a conformal transformation as a local unit transformation, we use a gravitational model in which the gravitational and the matter sectors are given in different conformal frames (or unit systems). It is relevant to the cosmological constant problem, namely the huge discrepancy between the estimated and the observational values of the cosmological constant in particle physics and cosmology, respectively. We argue that the problem arises when one ignores evolution of the conversion factor relating the two units during expansion of the Universe. Connection of the model with violation of equivalence principle and possible variation of fundamental constants are also discussed.

gr-qc↗

arXiv2018

Cosmological Constant in Chameleon Brans-Dicke Theory

We consider a generalized Brans-Dicke model in which the scalar field has a self-interacting potential function. The scalar field is also allowed to couple non-minimally with the matter part. We assume that it has a chameleon behavior in the sense that it acquires a density-dependent effective mass. We consider two different types of matter systems which couple with the chameleon, dust and vacuum. In the first case, we find a set of exact solutions when the potential has an exponential form. In the second case, we find a power-law exact solution for the scale factor. In this case, we will show that the vacuum density decays during expansion due to coupling with the chameleon.

hep-th↗

arXiv2018

Gravitational Coupling and the Cosmological Constant

We deal with a dynamical mechanism in which a large cosmological constant, as suggested by inflationary scenarios, decays due to expansion of the universe. This mechanism has its origin in the gravitational coupling of the vacuum density. We assume that the vacuum couples anomalously to gravity that is the metric tensor that appears the gravitational part is not the same as that appears the matter part as suggested by weak equivalence principle. Instead, the two metric tensors are taken to be conformally related. We show that this provides a dynamical mechanism which works during expansion of the universe. We also consider some observational consequences of such a gravitational model.

gr-qc↗

arXiv2015

Cosmological Exact Solutions in Some Modified Gravitational Theories

In a homogenous and isotropic cosmology, we introduce general exact solutions for some modified gravity models. In particular, we introduce exact solutions for power-law $f(R)$ gravity and Brans-Dicke theory in Einstein and Jordan conformal frames. In the Brans-Dicke case, the solutions are presented for both single and double exponential potentials in Einstein frame which correspond to power-law potentials in Jordan frame. Our analysis for extracting general exact solutions can also be generalized to those scalar-tensor theories in which the scalar field has an exponential coupling to Ricci scalar.

hep-th↗

arXiv2014

Notes on the Chameleon Brans-Dicke Gravity

We consider a generalized Brans-Dicke model in which the scalar field has a potential function and is also allowed to couple non-minimally with the matter sector. This anomalous gravitational coupling can in principle avoid the model to pass local gravity experiments. One then usually assumes that the scalar field has a chameleon behavior in the sense that it acquires a density-dependent effective mass. While it can take a small effective mass in cosmological (low-density environment) scale, it has a sufficiently heavy mass in Solar System (large-density environment) and then hides gravity tests. We will argue that such a chameleon behavior can not be generally realized and depends significantly on the forms attributed to the potential and the coupling functions.

gr-qc↗

arXiv2013

Non-minimal Gravitational Coupling of Phantom and Big Rip Singularity

We consider a non-minimal coupling of a perfect fluid matter system with geometry, which the coupling function is taken to be an arbitrary function of the Ricci scalar. Due to such a coupling, the matter stress tensor is no longer conserved and there is an energy transfer between the two components. By solving the conservation equation and applying the second law of thermodynamics, we show that direction of the energy transfer depends on the equation of state of the matter fluid. In particular, a phantom fluid should loose energy with expansion of the universe. This energy reduction can avoid the universe to end with a cosmic doomsday.

gr-qc↗

arXiv2012

On the Chameleon Brans-Dicke Cosmology

We consider a generalized Brans-Dicke model in which the scalar field has a potential function and is also allowed to couple non-minimally with the matter sector. We assume a power law form for the potential and the coupling functions as the inputs of the model and show that acceleration of the universe can be realized for a constrained range of exponent of the potential function. We also argue that this accelerating phase is consistent with a large and positive Brans-Dicke parameter. In our analysis, the potential plays a more important role with respect to the coupling function in dynamics of the universe as the latter does not contribute to any of the relations characterizing evolution of scale factor of the universe and the scalar field. However, we will show that the coupling function is closely related to magnitude and direction of the energy transfer between matter and the scale field. We use this fact and some thermodynamic aspects of the model to put some constraints on the coupling function. In particular, we argue that the second law of thermodynamics constrains direction of the overall energy transfer.

gr-qc↗

arXiv2012

Modified Gravity with a Non-minimal Gravitational Coupling to Matter

We consider modified theories of gravity with a direct coupling between matter and geometry, denoted by an arbitrary function in terms of the Ricci scalar. Due to such a coupling, the matter stress tensor is no longer conserved and there is an energy transfer between the two components. By solving the conservation equation, we argue that the matter system should gain energy in this interaction, as demanded by the second law of thermodynamics. In a cosmological setting, we show that although this kind of interaction may account for cosmic acceleration, this latter together with direction of the energy transfer constrain the coupling function.

gr-qc↗

arXiv2012

Cosmic Evolution in a Modified Brans-Dicke Theory

We consider Brans-Dicke theory with a self-interacting potential in Einstein conformal frame. We introduce a class of solutions in which an accelerating expansion is possible in a spatially flat universe for positive and large values of the Brans-Dicke parameter consistent with local gravity experiments. In this Einstein frame formulation, the theory appears as an interacting quintessence model in which the interaction term is given by the conformal transformation. In such an interacting model, we shall show that the solutions lead simultaneously to a constant ratio of energy densities of matter and the scalar field.

gr-qc↗

arXiv2012

Crossing Phantom Boundary in $f(R)$ Modified Gravity : Jordan Frame vs Einstein Frame

We study capability of $f(R)$ gravity models to allow crossing the phantom boundary in both Jordan and Einstein conformal frames. In Einstein frame, these models are equivalent to Einstein gravity together with a scalar field minimally coupled to gravity. This scalar degree of freedom appears as a quintessence field with a coupling with the matter sector. We investigate evolution of the equation of sate parameter for some cosmologically viable $f(R)$ gravity models in both conformal frames. This investigation (beyond mere theoretical arguments) acts as an operational tool to distinguish physical status of the two conformal frames. It shows that the two conformal frames have not the same physical status.

gr-qc↗